For the provision of steam for driving a steam turbine, feedwater is heated, evaporated and superheated under high pressure. This takes place, as a rule, in a plurality of steam generator stages. As a rule, this take place in a low-pressure steam generator stage, a medium-pressure steam generator stage and a high-pressure steam generator stage. The steam generator stages include a feedwater preheater or condensate preheater, in which the feedwater, which is supplied by a feedwater pump, is heated to boiling temperature before entry into a drum assigned to the respective pressure stage and is partially also evaporated. This is done in an evaporator, which is connected to the respective drum and in which saturated steam is generated. A superheater is connected to the steam region of the drum, for supplying heat to the saturated steam, without an increase in the pressure of the saturated steam. It is then subsequently conducted to a part of the steam turbine which corresponds to the respective steam pressure (low-pressure, medium-pressure and high-pressure turbine part).
In a combined gas/steam turbine plant, the supply of heat to the steam generator stages takes place by use of the hot exhaust gases from a gas turbine. In addition, in the medium-pressure steam generator stage, the exhaust steam obtained from a high-pressure turbine part is admixed, in turn, to the superheated medium-pressure steam. It is heated once more in an intermediate superheater, in order thereafter to be supplied to the medium-pressure turbine part. Alternatively to delivery to the corresponding steam turbine part, the steam may also be conducted into a steam turbine condenser, from where the water/steam circuit is closed by a condensate pump pumping to the condensate preheater again the condensate which is formed in the condenser.
For the provision of energy for generating gas for the gas turbine or additional heat for the steam generators of the steam turbine in a combined gas/steam turbine plant, fuel, for example natural gas, is burnt. In order to make this combustion effective, the fuel is previously heated. The preheating of the fuel takes place by way of a fuel preheater. For this purpose, for example, part of the already heated medium-pressure feedwater is branched off downstream of the medium-pressure feedwater preheater, before entry into the medium-pressure drum, is led past the fuel by the heat exchanger method and is delivered to the water/steam circuit again between the condensate pump and the condensate preheater. In this case, the fuel reaches temperatures of up to 200° C. Alternatively, medium-pressure feedwater is branched off even before entry into the medium-pressure feedwater preheater, fuel temperatures of about 140-160° C. being reached.
One disadvantage of this is that the fuel preheater must have a high pressure rating, with the result that it is sensitive toward temperature changes and is costly to produce and maintain. Moreover, even in modern gas turbine plants, the possible preheating of the fuel up to 200° C. can scarcely be utilized. This occurs since, in the case of higher preheating, there is, for example, the problem of what may be referred to as “combustion chamber humming”, so that it has hardly been possible hitherto to use the installed costly preheating systems.